Structurally-induced direct optical transitions in Ge-Si superlattices.

Using Schottky-barrier electro-reflectance spectroscopy, we have observed new direct optical transitions near 0.76 eV, 1.25 eV and 2.31 eV in an ordered Ge-Si superlattice. The superlattice structure consists of a sequence of 4 monolayers of germanium and 4 monolayers of silicon repeated five times in the (001) crystal orientation. The ordered superlattices were produced by molecular-beam epitaxy on (001) silicon in the commensurate strained-layer regime. The possible origins of these transitions are discussed in terms of modification of the electronic bandstructure by the sample structure.